Gamma cameras are primarily used to generate images of organs, bones or other tissues of the body. Typically, a low-level radioactive material is introduced into the body of a patient, which emits gamma rays that can be detected and measured by the gamma camera. Because the radioactive materials are formulated to collect temporarily in a specific part of the body, the emission of the gamma rays can enable a physician to review images of the areas of concern.
A typical gamma camera includes a scintillation crystal, which functions as a detector for the gamma rays from the patient's body. The crystal converts high-energy photons, such as gamma rays and X-rays, into visible light (lower-energy photons). When a gamma ray strikes and is absorbed in the scintillation crystal, the energy of the gamma ray is converted into flashes of light—a large number of scintillation photons—that emanate from the point of the gamma ray's absorption. A photo-multiplier tube (PMT), which is optically coupled to the scintillation crystal, detects a fraction of these scintillation photons and produces an output signal having an amplitude that is proportional to the number of detected scintillation photons. The gamma camera will generally include a two-dimensional array of PMTs, each capable of generating the proportional output signals. After a gamma ray absorption event, the outputs from the PMTs can be processed to determine the location of the absorption event.
In particular, the number of scintillation photons producing electrical signals in each PMT falls rapidly as the distance of the PMT from the point of gamma ray absorption, or event location, increases. The position of the event is typically calculated from an appropriately weighted centroid of the signals from the PMTs surrounding the event location.
Ideally, the total energy of a given gamma ray measured anywhere on the camera should have the same value. To achieve this principle, the gains of the PMTs must be matched (the camera must be “tuned”). Notably, the amplitudes of the signals derived from each PMT are proportional to two basic factors: 1) the number of scintillation photons detected by a PMT; and 2) the gain or amplification of the PMT.
As part of the tuning procedure, the gains of the PMTs are adjusted such that the sum of the output signals from all the PMTs are roughly equal in response to a fixed energy gamma event, regardless of the location of the event. The process of tuning a gamma camera relies on the knowledge of a contribution matrix. Prior art processes of determining the contribution matrix (using only a total energy signal) are tedious and time-consuming, and small matrix elements for them are poorly defined. Thus, in view of the difficulty in obtaining a contribution matrix, a generic contribution matrix is typically used for tuning conventional gamma cameras. A generic contribution matrix is normally an average of contribution matrices developed for several gamma cameras.
This generic matrix may be suitable for PMTs that are within the interior of the array in which they are positioned. Variation in the light collection at PMTs near the edges of the array, however, causes significant disparities in matrix elements for these PMTs. As such, the use of a generic contribution matrix bogs down the tuning process.